In wireless communication systems, multiple input multiple output (MIMO), is an antenna technique configured to improve the spectral efficiency and thereby boost overall system capacity. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N) on one end of the transmission system. MIMO systems can significantly increase the data carrying capacity of wireless systems. MIMO can be used for achieving diversity gain, spatial multiplexing gain and beamforming gain.
In new radio (NR), sometimes referred to as 5G, a user equipment computes channel estimates based on known pilots or reference signals from the 5G system, computes the parameters needed for channel state information (CSI) reporting and conveys this information to the network through the feedback channel. More particularly, the user equipment receiver estimates channel quality (typically signal-to-interference-plus-noise ratio, or SINR) from channel sounding, and computes a preferred precoding matrix indicator (PMI), rank indicator (RI), and Channel Quality Indicator (CQI) for the next downlink transmission.
For downlink data transmission, the demodulation reference signals (DM-RS) and the data are multiplied by the precoding matrix selected by the network device (e.g., gNode B) and transmitted. The user equipment receiver estimates the effective channel (the channel multiplied by the precoding matrix) and demodulates the data.
Codebook based precoding allows the receiver to explicitly identify the precoding matrix/vector that is to be used for transmission, based on a codebook. As an example, in the 3GPP NR standard, separate codebooks are defined for various combinations of the number of transmit antennas and the number of transmission layers; (the number of transmission layers is also referred to as rank information (RI)).
Finding the PMI/RI with larger numbers of antennas is a highly complex problem, involving many computations, and is based on an exhaustive search over codebook elements; the codebook size grows as the number of antennas increases. For example with four CSI-RS ports, the user equipment needs to search eighty precoding entities to find the rank information and precoding index. This can drain the user equipment battery life, increase power consumption, and/or consume significant memory and processing units at the user equipment.